The invention relates generally to polypropylene copolymers, and more particularly to polypropylene heterophasic copolymers modified with peroxide to improve the impact strength of such copolymers.
Polypropylene heterophasic copolymers are typically made up of three components. These include a polypropylene homopolymer, a rubbery ethylene propylene bipolymer, and a crystalline ethylene-rich ethylene propylene bipolymer. The typical heterophasic morphology of these polymers consists of the rubbery ethylene propylene bipolymer being dispersed as generally spherical domains within the semi-crystalline polypropylene homopolymer matrix.
Polypropylene copolymers are used in a variety of applications and products. In many instances, it is particularly important for the copolymers to have good impact strength characteristics. Polypropylene copolymers can be modified to improve the copolymers impact strength. This can be done through the use of elastomeric modifiers or with peroxides. When using elastomeric modifiers, the elastomeric modifiers are melt blended with the polypropylene copolymer, with the increased elastomer content typically contributing to a higher impact strength. Examples of elastomeric modifiers include ethylene propylene rubber (EPR) and ethylene propylene diene monomer (EPDM) rubber.
The use of peroxides to modify polypropylene polymers is also known. WO-95/11938 discloses a process of modifying copolymers by contacting them with a peroxide compound containing an activated unsaturated group and an acid group in the presence of a polymer reinforcing material, or prior to the addition of a polymer reinforcing material. The primary object of that invention was to modify copolymers in order to introduce an adhesion promoting functional group and to improve their properties. The resulting modified copolymers have improved impact strength, flexural strength, tensile strength and elongation at break, increased melt flow index and the other properties equal to those of the unmodified impact copolymers.
WO-97/49759 discloses a process for enhancing the melt strength of a propylene copolymer by the steps of mixing an initiator with the propylene copolymer at a temperature below the decomposition temperature; and then heating the mixture above the initiator decomposition temperature in order to decompose the initiator before the polymer has melted and in order to react the radicals created by the decomposition with the polymer.
WO-96/03444 discloses a process for modifying copolymers by contacting these with an organic cyclic ketone peroxide. Cyclic ketone peroxides have been found particularly efficient in the modification processes. They have been employed in the degradation of polyolefins, the cross-linking of polyolefins, the dynamic cross-linking of blends of elastomers and thermoplastic polymers, the grafting of monomers onto polymers, or the functionalization of polyolefins. The resulting modified copolymers had a larger melt flow index, a lower weight average molecular weight and a narrower molecular weight distribution than the starting copolymers, while keeping an adequate melt strength.
WO-96/20247 discloses cross-linked polymer compositions of propylene-ethylene copolymer and ethylene-xcex1-olefin copolymer prepared by melting and kneading the constituents in the presence of a radical forming agent, a cross-linking agent and eventually a peroxide inhibitor. These compositions were characterized by a high impact strength and a high flexural modulus.
EP-0,208,330 discloses a propylene polymer composition with increased whitening resistance and increased impact strength, obtained by addition of an ester, in the presence of peroxide, during extrusion.
While the aforementioned methods of modifying polymers are known, new techniques for yielding improved polypropylene heterophasic copolymers with high flow and impact strength are needed.
A method of preparing controlled rheology heterophasic polypropylene copolymers to increase impact strength of such copolymers is provided. The method comprises introducing a heterophasic polypropylene copolymer into an extruder along with a peroxide. The conditions within the extruder are such that the half life of the peroxide is increased by a factor of at least 2 compared to a half life of the peroxide under a normal extruder condition. The peroxide is allowed to degrade the polypropylene copolymer so that the copolymer obtained has a notched Izod impact strength that is increased by at least 50% compared to the same copolymer extruded with the peroxide under the normal extruder condition.
In one particular embodiment, a heterophasic polypropylene copolymer is prepared by introducing a heterophasic polypropylene copolymer into an extruder along with a peroxide under conditions wherein the temperature within the extruder is from about 150xc2x0 C. to about 215xc2x0 C., and wherein the half life of the peroxide is increased by a factor of at least 2 compared to a half life of the peroxide under a normal extruder condition in which the extruder temperature is greater than 215xc2x0 C. The peroxide is allowed to degrade the polypropylene copolymer so that the copolymer obtained has a notched Izod impact strength that is increased by at least 50% compared to the same copolymer extruded with the peroxide under the normal extruder condition. The degraded polypropylene copolymer has a melt flow index of greater than about 5 g/10 min.
In other specific embodiments, the polypropylene copolymer may have an ethylene content of from about 5 to 20% by weight, and may have an undegraded melt flow index of from about 0.05 g/10 min to about 5 g/10 min. The peroxide used may be a linear peroxide or a cyclic ketone peroxide, and may be introduced along with the copolymer in an amount of from about 0.005 wt % to about 0.5 wt %.